For application in high-power terahertz-wave generation, efficient wavelength conversion, ultra-high-speed light modulation device, and the like, organic materials having high nonlinear susceptibility and ultra-high-speed response which is said to be subpicosecond have been actively developed. With respect to an organic material 4-dimethylamino-N-methyl-4-stilbazolium tosylate (hereinafter also referred to as DAST), high-quality single crystallization techniques have been developed, and processes for producing the same have been proposed (see Patent Documents 1 and 2).
The DAST described above has high nonlinearity but is deliquescent, thus being problematic from the standpoint of processing or durability of a crystal. Specifically, because of the deliquescence, the DAST cannot be processed by normal optical polishing using water and abrasive grains; in addition, it deteriorates over time because it absorbs water vapor in the atmosphere, thus there have been difficulties in industrial applications.
On the other hand, BNA which is a novel organic nonlinear optical crystal, despite having nonlinear polarizability as high as that of the DAST described above, is not deliquescent and chemically stable, is easily subjected to optical polishing/processing, and shows excellent temporal stability even after use/storage in a normal environment. Thus, when considering industrial applications as a monochromatic terahertz wave-generating element or an optical device having broadband wavelength tunability in the future, BNA is a useful nonlinear optical crystal.
Regarding such a BNA crystal, it has been reported that the vertical Bridgman method was used to attempt single crystal growth from a melt to obtain a BNA crystal with a practical size (8×10 mm) (Non-Patent Document 1). Further, it has also been reported that the refractive index, absorption coefficient, and the like of the BNA single crystal described above were determined (Non-Patent Documents 2 and 3).